Rb-Sr analyses of 13 whole-rock samples of gneiss, quartz monzonite, and quartz diorite from the Precambrian basement complex in the Bighorn Mountains provide an isochron age of 2,849 ± 60 m.y. and an initial 87Sr/86Sr ratio of 0.7016 ± 0.0005. This age is interpreted as the time of regional metamorphism and metasomatism which produced the gneiss and the granitic rocks; independent isochron ages for both groups of rocks are not significantly different. The 2,850-m.y. age for the metamorphic event is consistent with other radiometric ages from the Bighorns and can be correlated with ages reported from several other mountain ranges in Wyoming.

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